Fiber-reinforced polymeric materials are used in the automotive industry to produce a variety of molded interior and exterior parts. Polymeric materials are desirable because, compared to sheet metal, they have a higher strength to weight ratio, are better able to resist corrosion and deterioration from weathering, and have more design flexibility.
Sheet molding compound (SMC) is a commonly used ready-to-mold, fiber-reinforced polyester material. SMC is prepared by dispensing an amount of cut fibers onto a thermosetting resin precursor composition that is carried on a film (usually of nylon or polyethylene). The fibers then disperse into and through the resin composition, and another sheet of film is placed on top of the fiber-resin mixture to sandwich them together and form a contained layer (or package) of SMC composite material. These packages are coiled on a take-up roll and stored to age, or mature, for a time suitable for the viscosity of the composite to reach a level sufficient for molding, typically between two to five days. When the SMC is ready to mold, molding charges are selected or cut from the aged packages and placed between facing, complementary, heated steel dies. Heat and pressure act on each charge to shape and cure it—to activate the polymerization of the thermosetting resin—resulting in solidification of the polymeric material and the formation of a molded SMC article.
A representative SMC resin precursor composition consists of approximately (on a fiber-free basis) 16.9 wt. % thermosetting resin, 2.6 wt. % styrene monomer, 13 wt. % low profile additive, 65 wt. % filler, for example, calcium carbonate, 1.5 wt. % thickener, 0.7 wt. % mold release agent, and 0.3 wt. % polymerization initiator. Reinforcing fibers comprise approximately 27 wt. % of the final SMC composite.
Exterior vehicle body panels are required to have smooth surfaces that are free of visual defects. However, reinforcing fibers in SMC composites tend to create cosmetic problems on the surface of molded SMC articles in the form of waviness, blistering, and porosity. Accordingly, molded SMC articles are typically coated with a suitably thick layer of a primer to fill-in and smooth over these imperfections, followed by one or more paint and gloss layers.
Electrostatic powder coating, instead of solvent-based coating, is a desired method of coating molded SMC articles because it reduces volatile organic compound (VOC) emissions and eliminates material waste. Electrostatic powder coating works by applying an electrostatic charge to the coating particles and to a surface of the article so that the particles accelerate toward the surface. This efficient technique reduces overspray and increases wrap-around of the coating particles, thereby reducing the amount of wasted coating material. Electrostatic powder coating requires the surface of the SMC article to hold an electric charge, but SMC composites are not inherently conductive. As such, a conductive coating composition can be applied to the surface of the SMC article so that the surface can receive and hold an electric charge, and can then be electrostatically powder coated.
Although powder coating of molded SMC articles is a desired and efficient method, this technique produces a surface defect known as “popping” when a layer of a powder primer is electrostatically applied to the SMC article, which is then heated to melt, flow and cure the primer particles. Popping is believed to result from the out-gassing of moisture from the SMC composite during the high-temperature (approximately 350° F.) baking of the powder primer layer. Out-gassing refers, generally, to the formation of bubbles or voids within the cured powder primer layer and on the layer's surface. These bubbles and voids result in a coated surface on the SMC article that is unacceptable for use in the automotive industry. Additionally, popping reduces the life of the SMC article by creating pathways that expose the composite core to environmental conditions.
It has been observed that the more moisture contained within the SMC composite material, the greater the extent of popping. Therefore, it is traditionally thought that such popping can be eliminated by reducing the amount of moisture absorbed by the SMC composite material or by sealing the surface of the molded SMC article to prevent the out-gassing of moisture when the article is heated. As such, conductive coating compositions have been formulated and employed to produce conductive coatings that are impermeable to moisture, for example, through the addition of inorganic platy fillers. However, such compositions have not been able to eliminate popping on the surface of powder-coated SMC composites, and may actually exacerbate popping, particularly when the powder-coated SMC is heated to temperatures above 350° F.
As of today, popping on the surface of powder-coated SMC articles continues to limit the practical use of SMC, especially in the automotive industry. And there is an industry-wide desire for a practical method of eliminating or reducing the extent of such popping.